Am I sinful if I gave my friend my phone and he watched foreign movies on it that contained immodestly dressed women, after I had advised him?

It is forbidden to give a phone to a friend who uses it to watch what is forbidden, because the guide to evil is like its perpetrator. Allah has prohibited cooperation in sin and aggression, as He said: "And do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty."